Configuration required for commercial use

Now it has been over 2 years since i am using your Guides for small scale mail servers. But this time i have come across a situation where i need to setup a large scale mail server with approx 1000 domains and approx 100000 concurrent users. Please suggest me the best suitable hardware reccomendation.

What i have planned for the above scenario

40 GB ram , 5TB SAN storage, 5-6 CPUs over godaddy or any other cloud
I prefer to use ubuntu 12.04 with dovecot and apache.

Over the cloud i have planned to use 3 different servers as mail server and distribute 333 domains on each server and i have written my own email client which will be running on fourth machine on the cloud.